Child Daily Hydration Calculator
Estimates a child's daily fluid needs in milliliters based on weight, activity level, climate, and illness. Use it to ensure your child stays properly hydrated in any condition.
Last updated: September 2026
Formula below · 3 sources (medlineplus.gov, healthychildren.org, aap.org) · Updated Sep 2026
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About this calculator
This calculator estimates a child's total daily fluid requirement using the Holliday-Segar maintenance volume as the baseline: weight is converted from pounds to kilograms (kg = lbs × 0.4536), then 100 mL/kg is allowed for the first 10 kg, 50 mL/kg for the next 10 kg and 20 mL/kg for every kg above 20 kg. Adjustments are layered on top: moderate activity adds 250 mL and high activity 500 mL; humid heat adds 500 mL, hot weather 300 mL and cool weather subtracts 100 mL; fever adds 400 mL and vomiting/diarrhoea 600 mL. Daily Fluid (mL) = Holliday-Segar(kg) + activity + climate + illness adjustments, rounded to the nearest millilitre. The total includes water from food. Babies under 6 months should get fluid only from breast milk or formula, and children who are vomiting need oral rehydration solution and medical advice if they cannot keep fluids down.
How to use
Suppose your child weighs 44 lbs, is moderately active, lives in a hot climate, and has no illness. Step 1 — Convert weight: 44 × 0.4536 = 19.96 kg. Step 2 — Holliday-Segar baseline: 1,000 + (19.96 − 10) × 50 = 1,498 mL. Step 3 — Add moderate activity: 1,498 + 250 = 1,748 mL. Step 4 — Add hot climate: 1,748 + 300 = 2,048 mL. Step 5 — No illness adjustment. Result ≈ 2,048 mL per day, roughly 8.5 cups including water from food.
Frequently asked questions
How much water should a child drink per day based on their weight?
The standard pediatric baseline is the Holliday-Segar rule: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg and 20 mL/kg above 20 kg. For example, a 20 kg child needs about 1,500 mL daily before accounting for activity or climate. This baseline is derived from standard clinical hydration recommendations for healthy children. Individual needs can vary, so always consult a pediatrician for medical guidance.
Why does activity level and climate affect a child's daily fluid requirements?
Children lose significant fluid through sweat during physical activity, especially in warm or humid environments. High activity can increase fluid needs by up to 500 mL per day, while a hot climate adds another 300 mL on top of the baseline. Humid conditions can add even more — up to 500 mL — because the body struggles to cool itself efficiently. Accounting for these factors helps prevent dehydration, which can impair concentration, mood, and physical performance in children.
When should I increase my child's fluid intake during illness?
Illnesses that cause fever or vomiting lead to accelerated fluid loss and dramatically increase hydration needs. A fever raises the body's temperature and metabolic rate, requiring an extra 400 mL per day to compensate. Vomiting causes direct fluid loss and warrants an additional 600 mL adjustment. If your child is both feverish and vomiting, these adjustments stack, making careful fluid replacement critical. Always seek medical advice if a child shows signs of severe dehydration such as dry mouth, sunken eyes, or reduced urination.